Method of operation for an oil-injected screw-compressor

ABSTRACT

This invention relates to a method at oil-injected screw-compressors for balancing axial forces at at least one of the rotors of the compressor, for sealing the gaps between rotor housing and rotor shafts and for cooling and lubricating the bearings of the rotor shafts. At the high-pressure end of the compressor oil under pressure of such magnitude is supplied to the bearing spaces (28a,28b) at the ends of both rotors that an oil flow inward to the compression space along the gaps (32a,32b) between the rotor shafts and the rotor housing is obtained for sealing against leakage from the compression space. Oil at this pressure further is supplied via a connection (42) from the bearing space (28b) of the female rotor to a pressure space (38) at the low-pressure end of the female rotor for balancing the axial force arising on the shaft end (31) of the female rotor on the high-pressure side due to the oil supplied to the bearing space (28b) of the female rotor.

This invention relates to a method at oil-injected screw-compressors forbalancing axial forces at at least one of the compressor rotors and forsealing the gaps between the rotor shafts and the rotor housing in orderto prevent leakage from the compression space of the screw-compressorthrough these gaps.

It was found very difficult in screw-compressors, at high pressuredifferences over the compressor to obtain a sufficient service life forthe axial bearings, by which the shafts of the rotors are supported.

In order to increase the service life of the axial bearings, it wasproposed to use balancing pistons such as shown, for example, in U.S.Pat. No. 3,161,349. This arrangement, however, has proved to involvegreat disadvantages, including a substantial oil leakage over the outerdiameter of the balancing piston. It was, therefore, difficult tomaintain the desired balancing pressure, and the leakage has causedlosses in the efficiency degree of the compressor. The way in which thebalancing piston had been built-in in the compressor, viz. on the rotorshaft located on the high-pressure side closest to the compressionspace, gives rise to the further disadvantage, that the balancingpressure is transferred along the rotor shaft inward to the rotor bodyand thereby acts also on the ring area formed by the end surface planeof the rotor body between the shaft diameter and the core diameter ofthe rotor. The force hereby arising on this ring area acts in adirection opposed to the desired balancing force and, thus, reduces thedesired balancing considerably. One object of the invention is toprovide a method of axially balancing the rotor shaft, whereby theaforesaid disadvantages of conventional arrangements are eliminated, andwhich method especially can be applied to screw-compressors operatingwith high pressure differences, of the magnitude 2 MPa and higher, overthe compressor.

A further object of the invention is to provide a method of theaforesaid kind, by which sealing against leakage from the compressionspace of the compressor along the shafts of the rotors is obtained.

A still further object of the invention is a method of the aforesaidkind to simultaneously effect lubrication and cooling of the bearingsprovided for the respective shaft.

The improved balancing in combination with the lubrication and coolingpossibility have been achieved in that the invention has been given thecharacterizing features defined in the attached claims.

The invention is described in greater detail in the following by way ofan embodiment thereof and with reference to the accompanying drawings,of which FIG. 1 is a horizontal section through a screw-compressorprovided with an arrangement according to the invention, and FIG. 2 is across-sectional view of typical rotors usable with the screw-compressorof FIG. 1.

The rotor housing 11 of the screw-compressor 10 includes a compressionspace in the form of two rotor barrels forming two intersecting bores,with a low-pressure port 12 at one end 13 and a high-pressure port (notshown) at the other end 14. In the rotor barrels, two meshing rotors,viz. one screw or male rotor 15 and one slide or female rotor 16, aremounted rotatably.

On the low-pressure side of the compressor, a radial bearing 17,preferably of roller bearing type, and an axial bearing 18, preferablyof angular contact ball bearing type, are built-in for supporting themale rotor 15. Outside said bearing package a balancing piston 19 islocated at the rotor shaft end 20 for balancing the main part of theaxial forces acting on the high-pressure end of the male rotor 15. Saidbalancing piston 19 is located in a pressure space 21, to which oilunder pressure can be supplied from the outside through an oil inletopening 22. The oil under pressure can be supplied by any suitablemeans, such as by an external oil pump or from the oil separatorconventionally used in the discharge pipe system of an oil injectedscrew compressor. At the outer diameter of the balancing piston amechanical seal 23 is located which ensures that a constant pressure ofthe oil supplied is maintained. In order to effect oil circulation forcooling this sealing 23 and for cooling and lubricating the bearingpackage 17,18, connections 24 are drilled from the pressure space 21outside the balancing piston 19 into the bearing space at the rotorshaft. The oil can continue to pass from here along gaps 25 between therotor shaft and the rotor housing into the compression space for sealingthese gaps 25, thereby eliminating leakage from the compression space.

The screw-compressor 10 is driven via the drive shaft 26 of the malerotor 15 which extends outward through the rotor housing 11 on thehigh-pressure side of said housing and is supported in a radial bearing27 located in a bearing space 28a. In this bearing space also amechanical shaft seal 29 of the drive shaft 26 is provided. The bearingspace 28a on the high-pressure side of the male rotor 15 is in directconnection with a bearing space 28b on the high-pressure side of thefemale rotor 16. In the bearing space 28b of the female rotor a radialbearing 30 is located for supporting the shaft 31 of the female rotor onthe high-pressure side. As can be seen, no special seals against thecompression space are built-in. Oil is supplied under pressure to thebearing spaces 28a,28b through a throttling 33, which is adjusted so asto deliver a pressure of the magnitude of the arithmetic mean value ofthe inlet and outlet pressure of the screw-compressor. This oil pressurecan also be supplied by any suitable means, such as by an external oilpump or an oil separator as previously mentioned. Since it has beenfound that the pressure around the shafts inside the compression spaceis at the mean value of the inlet and outlet pressures, an oil flow isensured through the bearings 27,30 along the gaps 32a,32b formed betweenthe rotor shafts 26,31 and the rotor housing into the compression space,whereby cooling and lubrication of these bearings 27,30 are obtained andat the same time gas leakage out of the compression space along therotor shafts 26,31 is prevented.

For supporting the female rotor 16 on the low-pressure side, a bearingpackage like the one for the male rotor 15 is built-in in the form of aradial bearing 34, preferably of roller bearing type, and an axialbearing 35, preferably of angular contact ball bearing type. Outsidethis bearing package, a balancing piston 36 is attached by screws (onescrew 43 indicated in the Figure)to the shaft end 37 of the femalerotor. (A corresponding attachment applies to the balancing piston 19 atthe shaft end of the male rotor). The balancing piston 36 is located ina pressure space 38, to which oil under pressure is supplied. At theouter diameter of the balancing piston 36 a mechanical seal 39 isprovided to ensure that a constant pressure of the oil supplied ismaintained. In order to effect oil circulation for cooling this sealing39 as well as for cooling and lubricating the bearing package 34,35,connections 40 are drilled from the pressure space 38 into the bearingspace at the shaft end 37 of the slide rotor. The oil can continue topass from here along gaps 41 between the rotor shaft and rotor housinginto the compression space for sealing these gaps 41 against leakagefrom the compression space.

For supplying oil to the pressure space 38 on the low-pressure side ofthe female rotor 16, a connection 42 is drilled axially along thecentral line of the female rotor, so that in the bearing space 28b ofthe female rotor and the pressure space 38 a common pressure isobtained.

The pressure space 21 for the balancing piston 19 of the male rotor 15is supplied with oil, the pressure of which corresponds to the outletpressure of the screw-compressor reduced by the pressure drop in the oilcooler and oil filter. The oil pressure in this pressure space 21, thus,is substantially higher than in the corresponding pressure space 38 onthe female rotor side. This higher pressure is desirable in view of thesubstantially higher axial forces acting on the male rotor 15 comparedwith those acting on the female rotor 16, the higher axial forcesresulting from the larger area on the male rotor 15 then on the femalerotor 16 on which the outlet gas forces act.

Due to the fact that the screw-compressor is driven from thehigh-pressure side of the male rotor, no free shaft journal area doesexist here and, thus, no additional axial forces from the bearing space28a on the high-pressure side are obtained. It was hereby possible tolimit the area on the balancing piston of the male rotor and to designit with the same size as on the female rotor side. It was herebypossible that both the bearing package and the balancing piston systeminclusive of the mechanical seals at the low-pressure ends on the maleand female rotor sides could be designed identical.

As regards the dimensioning of the connections 24 and 40 drilled fromthe pressure spaces 21 and, respectively, 38, the hole area obtained byone or more bores is calculated on the basis of available oil pressuredifference and oil viscosity, so that a suitable oil amount for coolingand lubricating the bearing package is obtained. This oil amountnormally is of the magnitude 5 liters/min per bearing package.

As regards the throttling 33, this is to be calculated so that the oilsupply therethrough slightly exceeds the oil amount calculated for theflow in the aforesaid connections. Hereby an oil supply along the gaps32a, 32b on the high-pressure side into the compression space always isobtained. It is, however, an essential and characterizing feature of theinvention, that a more accurate dimensioning of this throttling is notrequired, because an oil supply in excess of the aforesaid minimumamount implies only that the pressure in the bearing spaces 28a,28b andin the pressure space 38 of the female rotor increases relativelyinsignificantly, because due to the higher pressure the oil flow alongthe gaps 32a,32b on the high-pressure side increases simultaneously.

A further essential and characterizing advantage of the invention isthat, if the pressure according to the aforesaid increases in thebearing spaces 28a,28b, this does not effect the axial forces neither ofthe male rotor nor of the female rotor, because the ingoing shaft 26 isthe drive shaft, and the sealing area of the mechanical seal 29 islocated on the same level as the diameter of the rotor shaft extendinginto the compression space and, therefore, the pressure in the bearingspace 28a does not yield any axial force, but the axial forces on therotor shaft 26 substantially completely balance each other in this space28a. What has been stated above for the male rotor 15, applies inprinciple also to the female rotor 16, viz. that the axial forcessubstantially are independent of the pressure in the bearing space 28b,because the axial force obtained on the rotor shaft 31 in this space 28bis balanced substantially completely by the axial force obtained on thebalancing piston 36 on the opposite side of the female rotor, due to thefact that, as described above, the same pressure prevails on both sides,and that the pressure area on both sides substantially is of the samesize, because the diameter of the rotor shaft 31 on the high-pressureside is substantially the same as the diameter of the balancing piston36 on the low-pressure side.

The arrangement described above has brought about in an operationallyvery reliable way an oil circulation system for the cooling andlubrication of bearings and mechanical seals and an axial balancingsystem for both rotors, thereby rendering it possible to use simple andinexpensive bearings with a good service life. On the male rotor wherethe axial forces are high, the arrangement via the balancing pistonyields a balancing force, which increases with increasingcounterpressure in the compressor (the counterpressure being dependenton the outlet pressure, which corresponds to the pressure in space 21),the bearing forces and, thus, the bearing service life substantially areconstant. A change of the inlet pressure to the compressor does notaffect the axial forces acting on the male rotor from the balancingpiston or bearing space. As regards the female rotor, the axial forcesfrom the balancing piston and bearing space are not affected, either, bychanges in the inlet or outlet pressures of the compressor.

What we claim is:
 1. A method at an oil-injected screw-compressor forbalancing axial forces at at least one of the rotors of the compressor,for sealing the gaps between the rotor housing and the shafts of therotors and for cooling and lubricating the bearings of the rotor shafts,where a rotor housing includes a compression space in the form of tworotor barrels defined by two intersecting bores with a low-pressure portat one end and a high-pressure port at the other end, and two meshingrotors, viz. a male rotor and a female rotor, mounted rotatably in therotor barrels, which compressor is driven on the shaft of the male rotoron the high-pressure side, characterized in that at the high-pressureend of the compressor oil under pressure is supplied to bearing spacesat the ends of both rotors, that an oil flow inward to the compressionspace along the gaps between the rotor shafts and the rotor housing isobtained for sealing against leakage from the compression space, andthat oil is supplied via a connection from the bearing space of thefemale rotor to a pressure space at the low-pressure end of the femalerotor for balancing the axial force arising on the shaft end of thefemale rotor on the high-pressure side due to the oil supplied to thebearing space of the female rotor.
 2. A method as defined in claim 1,characterized in that the oil is supplied to the bearing spaces througha common inlet conduit, and that the two bearing spaces are in directconnection with each other.
 3. A method as defined in claim 1,characterized in that the supply of oil from the bearing space of thefemale rotor to the pressure space is effected via a bore extendingcentrally through the slide rotor shaft.
 4. A method as defined in claim1, characterized in that roller bearings provided for supporting therotor shafts in the bearing spaces are cooled and lubricated by the oilsupplied to the bearing spaces.
 5. A method as defined in claim 1,characterized in that for preventing axial forces from being supplied tothe shaft of the male rotor due to the oil supplied to the bearing spaceof the male rotor, the oil acts on a mechanical seal located about thedrive shaft in such a manner that the sealed surface has about the samediameter as the diameter of the rotor shaft where the shaft passesthrough the rotor housing.
 6. A method as defined in claim 1,characterized in that for axial balancing the male rotor, oil underpressure is supplied to a pressure space adjacent to the shaft end ofthe male rotor on the low-pressure side.
 7. A method as defined in claim6, characterized in that for balancing the shaft ends of the male rotorand, respectively, female rotor, balancing pistons located in thepressure spaces are pressed against the shaft ends, and that the outerdiameters of the balancing pistons are sealed by mechanical seals sothat the oil pressures supplied to the pressure spaces are maintained.8. A method as defined in claim 7, characterized in that for supportingthe rotor shafts on the low-pressure side, the shaft ends of the maleand female rotors are supported in roller bearings and angular contactball bearings.
 9. A method as defined in claim 8, characterized in thatfrom the pressure spaces oil is supplied via connections equipped withthrottlings to the bearings for lubricating and cooling the same.